Radio communication systems of today typically employ a modulation and coding scheme in which a data-carrying signal is superimposed on or mixed into a propagating carrier signal.
For some communication systems, including a GSM (Global System for Mobile Communication) or GPRS (General Packet Radio Service) system, the sole choice of available modulation and coding scheme has been GMSK (Gaussian Minimum Shift Keying). GMSK is a kind of constant-envelope phase modulation, where transmitting a zero bit or a one bit is represented by a change in the phase of the signal. Thus, every transmitted symbol represents one bit.
Introduction of the EDGE (Enhanced Data rates for GSM Evolution) technology into a GPRS system provides another modulation and coding scheme to be employable for radio communications, namely 8-PSK (8-state Phase Shift Keying). 8-PSK enables reuse of the channel structure, channel width and the existing mechanisms and functionality of the GMSK-using GPRS system. However, 8-PSK enables higher bit rates per time slot than those available for GMSK. 8-PSK is a linear method that uses phase modulation, in which three consecutive bits are mapped onto one symbol. Although the symbol rate remains the same as for GMSK, each symbol now represents three bits instead of one, thus increasing the total data rate by a factor of three.
In addition to the above mentioned 8-PSK method, an even higher order modulation and coding scheme is being discussed, namely 16QAM. This, when introduced, will enable even more available MCS modes.
Present day Enhanced GPRS (E-GPRS) systems having access to both GMSK and 8-PSK modulation can use nine different modulation and coding schemes (MCS:s), MCS-1 to MCS-9. The lower four coding schemes (MCS-1-MCS-4) use GMSK whereas the upper five (MCS-5-MCS-9) use 8-PSK. These nine MCS:s use different error correction and consequently are adapted for usage under different environment conditions. Generally, in good radio environments, a more aggressive (less error correction, possibly also 8-PSK associated MCS) coding scheme can be used to provide a higher user data rate, whereas with a poor radio link environment a coding scheme with more error correction (possibly also GMSK-associated MCS) and lower user data rate is typically used.
The E-GPRS system also employs a link quality control (LQC) functionality, usually denoted link adaptation. Link adaptation uses radio link quality measurements from a mobile terminal to select the most appropriate modulation and coding scheme for transmission of subsequent data packets or data blocks to the mobile terminal. Such a measurement report from the mobile terminal typically includes only link quality measurements e.g. BEP (Bit Error Probability) for the modulation that has been used since a last measurement report.
In GSM/EDGE, RLC (Radio Link Control) blocks, comprising the actual payload or data blocks to be transmitted, are sent over two or four radio bursts depending on the coding scheme. For MCS-1-7, both the header and the RLC block(s) are interleaved over four bursts. (A radio block comprises four bursts or sub-blocks.) For MCS:s-8-9 the header is interleaved over all four bursts of a radio block, but the two RLC blocks are separated on two bursts each.
The different modulation and coding schemes also have different diversity properties. The MCS:s with robust coding (e.g. MCS-1 and MCS-5) benefit from as much diversity as possible (e.g. use of frequency hopping). Coding schemes with weaker coding on the other hand (e.g. MCS-4 and MCS-9) benefit from as little diversity as possible (e.g. by not using frequency hopping and try to get the bursts within a radio block as correlated as possible). [1], [2].
Typically, the choice of MCS is based on information from the previously mentioned measurement reports. These reports usually contain estimates on the distribution (mean and standard deviation) of burst bit error (BER) over a radio block, and are typically measured and reported by each mobile terminal to its associated base transceiver station or base station controller.
For the case of multiple time slots and especially with multi-carrier, there is a large number of possible ways to theoretically choose a set of bursts (burst constellation) on which to transmit an RLC block, based on the discussion above. Today, this set of bursts is fixed to one carrier, one timeslot and sequentially in time, which means the blocks are not necessarily transmitted in the most efficient manner.
If a radio block is pre-allocated on fixed allocation of bursts this also implies that the only way to optimize e.g. throughput is by attempting a correct MCS choice. Furthermore, other requirements like short transmission time intervals (TTI) are not possible to address dynamically at all.
In general, for channel coding frequency hopping has a positive effect on the decoding capability due to interference and frequency diversity. However, if the data to be transmitted has a high coding rate or even is virtually uncoded (which is the case for MCS-4 and MCS-9 in GSM/EDGE), the chance of correctly received data relies on good link quality throughout a complete block or frame. Frequency hopping implies that bad quality is distributed between blocks or frames and is hence not desirable for these coding schemes.
Consequently, there is a need for improved link adaptation and associated selection of optimal modulation and coding schemes.